Cell death via the necroptosis pathway has been implicated in many human pathologies and is widely considered to have evolved as an innate immunity mechanism. Recent data have revealed rapid evolution of the pathway, with important mechanistic differences apparent between mouse and human pathways. At the heart of these interspecies differences are co-evolved signaling cassettes comprising the protein kinase, RIPK3, and the terminal effector in the necroptosis pathway, the pseudokinase MLKL.
We propose that pathogen-encoded proteins targeting the pathway, such as orthopoxvirus MLKL (vMLKL) orthologs, have driven this rapid evolutionary divergence. Expression of vMLKL in human and mouse cell lines led to abrogation of necroptotic cell death and hallmarks of necroptosis, such as cellular MLKL phosphorylation and cytokine release, without impacting apoptosis. vMLKL comprises solely a pseudokinase domain and lacks the N-terminal executioner domain present in cellular MLKL. This befits its function as a mimic of host MLKL to inhibit necroptosis by binding the RIPK3 kinase domain to thwart both necrosome assembly and engagement and phosphorylation of host MLKL. These data support the idea that the ancestral origins of necroptosis lie in host defense with subsequent evolutionary loss of necroptotic effectors from a subset of animal genomes having been driven by pathogen subversion of the pathway.